What it means

A 426 Upgrade Required response indicates the server refuses to perform the request using the current protocol but might be willing to do so after the client upgrades, often to TLS or a newer HTTP version.

Common causes

• The server requires HTTPS but the client used HTTP.
• The server requires a newer protocol version.

How to fix it

• Upgrade the request to the protocol specified in the response's Upgrade header.
• Use HTTPS instead of HTTP if required.